Regolith geology and geochemical exploration around the Stellar and Quasar gold deposits, Mt Magnet, Western Australia

Abstract

The Boogardie Synform, at Mt Magnet, in the NE Yilgarn Craton of Western Australia, is difficult to explore due to structural complexity in the basement, a variably truncated, residual regolith beneath a complex, transported overburden and high geochemical backgrounds from numerous mineralized settings. The distributions of Au and associated elements in the regolith around the Stellar and Quasar deposits, within the Boogardie Synform, have been studied to identify optimum sample media and an analytical suite for continuing exploration of this district. Regolith-landform relationships and geochemical dispersion in the district were established by mapping the regolith of an area of 25 km 2 around the mines and investigating drill cuttings and pit exposures.Mining and drilling at both sites revealed a partly eroded lateritic regolith incized by a fluvial palaeochannel, all concealed beneath 5-10 m of polymictic colluviumalluvium. The sediments have all undergone post-depositional weathering. The Au mineralization is poor in As and Sb, but there are weak anomalies in Bi and Pb in the upper part of the weathered basement. There is no expression of the mineralization in the palaeochannel sediments or colluvium, nor between nearly adjacent layers within the colluvium and its overlying lag. Comparison of data from samples taken from the top of the basement with those from the unconformity, or interface, between basement and colluvium, indicates lower order but broader (200 m) and more consistent anomalies in the interface for Au, Bi and Pb. Useful anomalies can be improved using additive indices of Bi, Pb and Zn. The interface sample is, therefore, the preferred sample medium in this area of buried, truncated regolith, except where there are palaeochannels. Where there has been less regolith truncation and a buried, ferruginous duricrust is present, as at Stellar, duricrust is the optimum sample medium.Elevated concentrations of Cr, Ni, Cu, V and, possibly, Zn in the basement regolith indicate mafic-ultramafic rocks. The saprolite and mottled zone on the basement rocks have a significantly greater Cr/Fe ratio than the transported materials. Regolith differentiation can be improved by robust canonical variate analysis; the most useful elements appear to be Al, Fe, Ni, Cr, Ga, Y, Zn, Th and Cu.

Regolith geology of the Yilgarn Craton, Western Australia: Implications for exploration

“…The upper surface of dolomite is close to horizontal, except towards the centre of the channel where some collapse has occurred. A similar stratigraphy in palaeochannel sediments has been observed in a number of sites, including Stellar (Mt Magnet) (Robertson et al 1994) in the northwest. At Stellar, the palaeochannel sediments are up to 18 m thick and consist of a grey, plastic clay and minor sandy clay (smectite and kaolinite).…”

“…Numerous palaeochannels occupy the lower parts of the landscape in the northern Yilgarn (Anand et al 1991b, 1999a; Robertson et al 1994; Varga et al 1997; Johnston et al 1999; Wildman & Compston 2000), for example, in the Yandal greenstone belt (Figure 49). The stratigraphy and post‐depositional weathering is broadly similar to that described by Kern and Commander (1993) for the Roe palaeodrainage in the Kalgoorlie region, namely a lower basal sand in the axis of the channel, overlain by a thick sequence of clay‐rich sediments, strongly overprinted by ferruginous mottling.…”

“…Kanowna Belle, Wombola). This unit has been studied from exposures in the Stellar pit, Mt Magnet (Robertson et al 1994). Here, there are three facies, with each being laterally continuous across the pit face.…”

“…Cyclonic prospect near Bronzewing (Ely 2000)] or more commonly saprolite [e.g. Kanowna (Dell 1992; Ladhams 1994) Mt Magnet (Robertson et al 1994)]. Lower units may contain detrital pisoliths and nodules (Ladhams 1994; Anand 1995).…”

Buried Inset‐Valleys in the Eastern Yilgarn Craton, Western Australia: Geomorphology, Age, and Allogenic Control

“…Buried inset-valleys containing palynologically dated or lithostratigraphically correlated Eocene sediments at their base also occur west of the continental divide on the Yilgarn Craton (Smyth and Button 1989; Waterhouse et al 1994;Robertson et al 2001) and Capricorn Orogen (Robertson et al 1999), along the south coast within the onshore Bremer Basin (Comet Resources NL, unpub. data, 1999), and around the northeastern margin of the Eucla Basin in South Australia (Benbow et al 1995;Alley et al 1999;Hou et al 2003aHou et al , 2003b fig.…”